Essential tremor is the most common movement disorder in clinical medicine and is hypothesized to be a pathologic expression of a normal, physiologic oscillator located within the central nervous system. This hypothesis is supported by the similarities between mild essential tremor and the B- to 12- Hz component of physiologic tremor and by the observation that tremor frequency decreases as essential tremor worsens, resulting in a broad frequency range of 5 to 12 Hz. This range of frequencies is difficult to explain on the basis of popular linear servo control theory and putative un- stable stretch reflex pathways. The dynamical complexities of essential tremor are best explained by its nonlinear properties. Essential tremor exhibits nonlinear, limit-cycle behavior. The interaction of this oscillator with the segmental stretch reflex and limb mechanics could explain the rich spectrum of dynamical behavior observed in patients with essential tremor. The general aim of the proposed grant is to systematically study the nonlinear dynamics of essential tremor during experimentally-imposed stretch- reflex disturbances and during voluntary movement. A computer- controlled torque motor is programmed to deliver torque-pulse perturbations and sinusoidal torque and displacement forcings to the wrists of essential tremor patients and adult controls. Wrist movement, wrist torque and forearm electromyograms are recorded. Phase response curves are computed for the torque-pulse data to determine the extent to which sensory feedback is capable of resetting the essential tremor rhythm. The wrist and EMG responses to sinusoidal torque and displacement forcing are analyzed by phase-plane and describing-function techniques. The nonlinear dynamical phenomena of subharmonic oscillation, frequency entrainment, asynchronous quenching and chaos are sought in an effort to explain 1. the frequency and amplitude characteristics of essential tremor and 2) the relationship between mild essential tremor and the 8- to 12-Hz physiologic tremor. The interaction between essential tremor and voluntary ballistic movement is studied in patients instructed to execute ballistic wrist flexion or extension in a simple reaction-time paradigm. The dynamical relationships between tremor and the voluntary oscillations of cursive handwriting are also examined.